Blender gpu renders turn completely gray
So I have an AMD Radeon HD 7800 series video card. It worked wonderfully for rendering gpu in Blender up  until 2.78 came out. Once 2.78 came out, I lost complete ability to render with GPU up until 2.78b. It kept giving me errors when I would try to render it. Once 2.78b came, the errors stopped, but I had the same gray render that you have. I decided to boost the light significantly, and try rendering again. It works, you can now see the object with the boosted light. You'll have to boost your lighting to use current version GPU for AMD, and it'll have significantly more noise. It will also be way over exposed if you render this same scene with cpu with the boosted light settings. I hope they fix it soon.